The accurate and rapid intra-oral measurement of dental surfaces for many purposes including the production of prosthodontics or dental parts has been a goal of Dental Science for some time.
A system developed by Francois Duret et al. combines holographic moire techniques to produce an array of three dimensional points that represent a single-view image of a tooth. The hand held optical probe consists primarily of a laser diode and a CCD photo sensor in order to capture dental images. The spatial resolution of the dental images produced with this technique is about 20 μm. Images from different views are then interactively combined, and a rigid theoretical tooth is fitted to the points in order to reconstruct the tooth analytically. With this system, a dental practitioner can personalize the anatomy of a tooth. Further, this system allows design of a custom crown for a particular tooth as well as subsequent milling for immediate fabrication and restoration. The final restoration error is about 80 μm and can produce a posterior crown in less than an hour. However, as mentioned this system requires the use of a laser in the vicinity of a patient's eyes.
U.S. Pat. No. 4,575,805 describes a system called CEREC (Ceramic Reconstruction). According to this patent, the intra-oral scanner incorporates a light emitting diode and lens system to illuminate the cavity of the tooth. The light rays pass through a set of ruled lines, casting stripe patterns on the prepared cavity. A CCD camera is used to record the stripe pattern in a 12.8 mm3 volume. Due to the limitations on the width of the ruled lines, spatial resolution is quite low. To increase the spatial resolution, a mechanism was introduced that requires multiple frame images. Using a piezo motor, the ruler is moved to four fixed and offset locations, allowing the CCD camera to take an image at each of the four locations. The number of measurements is thus quadrupled. In this arrangement, the system loses the capability of taking a complete 3 dimensional measurement in a single snapshot, and the design of the system becomes fairly complicated.
Rekow developed a system known as the Minnesota System. The raw image of a tooth is acquired using a standard 35 mm camera through a 10 mm diameter single rod lens magnifying laryngopharyngoscope. A prism system at the distal end of the rod lens permits the field of view to be 90 degrees. A number of views are used to ensure that complete information is obtained and to minimize the likelihood of blur caused by patient movement. Fiber optics provides the illumination necessary to capture the stereos images, or slides, that are taken on standard photographic film. The slides are then digitized in 4096×4096 resolution. Stereo correspondence algorithms are used to produce three dimensional measurement data. The aim of this system was to produce a low cost high resolution three dimensional measurement. It does not, however, take advantage of the rapid advances in the field of machine vision and analysis.
While each of the above prior art systems has merit in the measurement of dental structure, each have shortcomings of one type or another. Most require multiple imaging which in turn requires that the patient maintain a fixed position for a long period of time. In addition, most of the above systems lack adequate resolution or expose the patient to undesirable radiation such as a laser.
U.S. Pat. No. 5,675,407 to Geng issued Oct. 7, 1997 describes a novel three-dimensional surface profile measuring technique that is able to acquire full frame, dynamic 3-D images of objects with complex surface geometries at high speed. By “full frame 3-D image” is meant that the value of each pixel (i.e. picture element) in an acquired digital image represents the accurate distance from the camera's focal point to the corresponding point on the object's surface. The (x,y,z) coordinates for all visible points on the object surface are supplied by a single 3-D image. By “acquiring dynamic 3-D images at high speed” is meant, that a camera of the type described in U.S. Pat. No. 5,675,407 is able to capture a full frame 3-D image in one snapshot, i.e. within one exposure time of its imager device (for example, within one millisecond), and can obtain a stream of such 3-D images at a sustainable speed of at least 30 frames per second.